1. Field of the Invention
The present invention relates to the vaporization of liquids and the pressurization of vapors in capillary force vaporizers. More particularly, the invention relates to improved capillary force vaporizers, as well as capillary force vaporizer modules and systems for producing vapor for a variety of applications.
2. Description of the Pertinent Art
Many applications utilize gases that have been generated from liquid sources. Vaporization devices have been designed to vaporize liquids and release the resulting vapor under pressure. In applications in which a pressurized vapor stream is desired, prior art devices generally require that liquid be supplied to the device under pressure, or that the vapor is otherwise pressurized by external means. For example, in pressurized boiler systems, liquids are generally required to be supplied under at least as much pressure as that of the produced vapor. Pressurized liquid sources are usually inconvenient to use, heavy to transport, potentially explosive, and prone to leakage. It is desirable, for many applications, to produce pressurized vapor streams directly from liquids that are either at or near atmospheric pressure. One class of devices that achieve this goal are known in the art as capillary pumps, capillary vaporization modules or capillary force vaporizers. These devices all generate pressurized vapor directly from unpressurized liquid by applying heat to cause liquid boiling within a capillary member, and by at least partially constraining the evolved vapor to allow pressure to increase, exiting the device through one or more orifices as a high velocity jet. Other features, which these devices have in common, are that they all are thermally powered, compact, and generally have no moving parts, thereby offering certain advantages over other techniques used for liquid vaporization and vapor pressurization. Capillary pumps, capillary vaporization modules, capillary force vaporizers and devices in which they may be found are variously described in U.S. Pat. Nos. 5,692,095; 5,870,525; 6,162,046; 6,347,936; 6,585,509 and 6,634,864.
While a number of the prior art devices mentioned above offer advantages over alternative liquid vaporization technologies, the devices were found to have certain limitations. First, some prior art capillary force vaporizers occasionally exhibited poor mechanical reliability. The devices relied upon a typically thin and brittle coating material for an impermeable peripheral sealing member. The sealing member, in association with the liquid filled porous member in which vaporization takes place, allows pressure to increase within the device and also provides structural integrity to hold the various components together in intimate contact. Under thermal gradients, internal pressure and temperature cycling experienced during certain device operation, the peripheral seal component is prone to cracking, occasionally resulting in reduced performance or premature device failure. As thermal stresses tend to increase with size, cracking was more common in larger devices. Attempts to generate larger capillary force vaporizers to meet higher output flow rate requirements needed for some applications therefore met with limited success.
Other disadvantages of certain prior art capillary force vaporizers were the relatively slow response time experienced in responding to changes in input heat or power, and limited energy efficiency. The slow response time and limited energy efficiency are thought to be a consequence of having to conduct heat from a heat source external to the device and far from the vaporization zone. This results in having an excess mass in contact with the vaporization member that must be heated or cooled before a change in the vaporization rate can take place, thus causing undesirable energy loss to the surrounding environment. Excess heater temperatures and resulting shorter lifetimes; fabrication issues in that additional surfaces are required to be smooth; and the extra complexity of requiring additional parts are other disadvantages associated with prior art capillary force vaporizers.